Abstract
Much of the information that exists today about Transverse Momentum Dependent Parton Distri- bution Functions (TMDs) comes from SIDIS measurements with unpolarised and polarised beams and targets where they appear convoluted with fragmentation functions (FFs). Drell-Yan (DY) measurements are com- plementary to those by SIDIS experiments, as they allow to measure convolutions of only Parton Distribution Functions (PDFs) without involving FFs. Moreover, given the T-odd character of both Sivers and Boer-Mulders functions, the sign of these TMDs is expected to be reversed when observed from SIDIS or from DY. Measure- ments of SIDIS were performed by Compass in the period 2002 to 2007 and in 2010, using a naturally polarised μ + beam and a solid state target polarised either longitudinally or transversely with respect to the beam direction. Now the COMPASS Experiment has the unique opportunity to access TMDs from single-polarised Drell-Yan processes as well, in the same kinematical domain of the SIDIS data and with the same experimental setup; it will allow to test the TMDs universality and the sign change prediction for the first time. We present here the upcoming polarised DY measurements at COMPASS .
Highlights
Much of the information that exists today about Transverse Momentum Dependent Parton Distribution Functions (TMDs) comes from SIDIS measurements with unpolarised and polarised beams and targets where they appear convoluted with fragmentation functions (FFs)
By performing the measurements of target spindependent asymmetries in DY reaction and comparing the results with ones obtained in SIDIS, we will be able for the first time ever to verify the universality of TMD approach for the description of these reactions
A clear Sivers asymmetry is visible in all Q2 :x range for positive hadrons
Summary
The main goal of COMPASS DY programme [2] is to measure the process π− p↑ → μ+μ−X with a 190 GeV π− beam scattered off a transversely polarised proton target (N H3 ). By performing the measurements of target spin (in)dependent asymmetries in DY reaction and comparing the results with ones obtained in SIDIS, we will be able for the first time ever to verify the universality of TMD approach for the description of these reactions. This would be a crucial test of QCD in the non-perturbative regime. A non-zero effect is observable for negative hadrons at high Q2 :x values
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